Ligaments are viewed as passive structural elements and therapeutic concepts have developed that are consistent with this view. Compelling evidence suggests an alternative view, one in which mechanoreceptors located in ligament provide information for motor control. This 'neurosensory' view carries implications for treatment of ligament injury, and may help account for the currently variable recovery patterns observed following ligament injury and treatment. However, fundamental data concerning what is signalled by ligament afferent fibers during posture and movement is virtually nonexistent. The effects of this afferent discharge on motor control is also unknown. Three complementary studies are proposed concerning the discharge properties of anterior cruciate ligament (ACL) afferents. In Specific Aim 1 an isolated ACL preparation (50 knees) will be used to characterize the discharge properties of ACL afferents in response to controlled mechanical stimulation of the ligament. The transduction characteristics of the ligament and afferents will be determined. Studies in Specific Aim 2 will focus on the discharge properties of ACL afferents in the intact knee (n=50) under more physiological conditions; during knee movement. Results and interpretation in the first and second study are complementary. In Specific Aim 3, the effects of precise mechanical stimulation of the isolated ACL on segmental reflex pathways will be evaluated to ascertain their influence in modulating leg muscle activity. It is hypothesized that increasing strain or rate of strain in the ACL will facilitate knee extensor muscle activity, thus reinforcing knee extension during locomotion, rather than inhibiting extension (protection). This project is the first step towards long-term goals aimed at 1) determining the contributions of ligament afferent signals to posture and locomotion, 2) identify which contributions are lost after injury, 3) discerning neurally- from structurally-impaired injured patients, and 4) suggesting treatments which best rehabilitate the neurally-impaired patient.